Electroless Ni/Pd/Au plating for semiconductor package substrates (I) - Influence of the electroless Ni plating thickness on the solder ball joint reliability

Abstract

We have adopted an electroless Ni/Pd/Au plating for the surface finishing of package substrates which ensure the same Au wire bonding reliability and high impact resistance as the conventional electrolytic Ni/Au finishing at the solder-ball joint. To utilize future substrates having fine wiring spaces of less than 20 μm, we investigated the minimum thickness of the electroless Ni fllm ensuring impact resistance by means of the high-speed solder ball shear test method. We found that the minimum electroless Ni film thickness is 1 μm with a Sn-3Ag-0.5Cu solder ball after heat (seven times 252°C peak N 2 reflow or 1,000 h @ 150°C in air). We also found that the formations of voids and the grain miniaturization of intermetallic compound (IMC) near the interface of the ball-pad and the solder were the causes of brittle fracture which is the same failure mode in the drop test of mobile equipment.